B737 Captain reported a possible fuel leak during cruise climb resulted in return to departure airport.

Narrative

CA (me) was PF; FO was PMClimb-out passing through FL280(ish). IMBAL annunciation appeared with an amber fuel quantity on right engine.Left quantity was around 6.8; right quantity was 5.7 and decreasing rapidly.I immediately looked up at the fuel panel. Main wing tank pumps were all on; center tanks were off (no fuel in center); but the crossfeed was on and the blue Valve Open light was on*. I turned the crossfeed valve off immediately and the blue light went out; and we watched the right fuel quantity to see if it would stop decreasing. We discussed that even with the crossfeed valve open and all four pumps on; an imbalance should develop slow; and the fuel quantity should change slower than what we were observing. The right fuel quantity continued dropping rapidly. I asked the FO to open the QRH while we watched for another 3-4 minutes to see if the quantity would slow down. I briefly read the first page of the FUEL LEAK ENGINE checklist quietly and double checked the fuel panel.We were approaching FL370 when I asked the FO to request a turn back to ZZZ. We discussed the nearest field; and we appeared to be halfway between ZZZ and ZZZ1. I elected to return to ZZZ. The left fuel tank was still approximately 6.7; and the right was now down to about 5.0 and still decreasing quicker than the left.ZZZ Center initially gave us a heading of 320 and ask what the nature of our return was. We (FO) answered that it was a possible fuel leak. Center asked if we were requesting priority; and we answered YES. I then told the FO that I was transferring the aircraft to him. He would act as PF; and I would run the QRH.Since the FO had already reviewed some of the QRH quietly; I asked him what page/checklist he was in. He answered that he was in the IMBAL checklist. I found that page; and began officially" running the IMBAL checklist. The engine fuel flows on the 1 & 2 engine were comparable. I decided that a fuel leak was suspected due to the rate at which the fuel quantity was changing. This directed us to run the FUEL LEAK ENGINE checklist. After running the first four steps to verify the fuel panel switch positions (again); we read steps 5 and 6 and did not need to "time" the fuel quantities; as the right changed by >100 pounds per minute (not 500 per 30 minutes as the checklist states). Center gave as an initial descent to FL300(I think) and another vector; followed by direct ZZZ. As we were established in the descent; we both noticed that the fuel leak had significantly slowed or stopped. It made sense to us that with little to no demand on the engine; the fuel leak would slow dramatically.With a fuel leak confirmed; we were in a descent direct to ZZZ. We discussed the decision to shut down the number 2 engine. I decided to leave the engine running at idle at least until we were closer to ZZZ unless we saw additional indications of trouble; and the FO agreed. I paused the FUEL LEAK ENGINE checklist at step XX. (I read the remaining steps but did not accomplish them.)At this point the FO was flying us back to ZZZ; reprogramming the FMS for the arrival; and coordinating with ATC. He briefly answered an ACARS from the dispatcher confirming our return to ZZZ.I began running the non-routine landing considerations checklist. Although the dispatcher was aware we were returning; I advised them of the urgency. I then advised the Flight Attendants of a "Precautionary Landing" and gave the instructions associated with that. I then made a PA to the passengers advising them of our return to ZZZ due to a "fuel issue" and advised them that there would be safety vehicles present.We informed Center that we were requesting RWY XXC and requested ARFF. The FO had loaded the FMS; and I set the ILS frequency and course and reviewed the ILS XXC quietly. I offered the FO the approach and he accepted. FO remained the PF until landing. The weather was VFR and we briefed the visual to XXC. I accomplished theDescent checklist and asked the FAs to prepare the cabin for landing. The FO and I discussed clearing the runway versus stopping on the runway. I decided to stop on the runway to allow room for the ARFF to drive around the wingtips for inspection. We continued to monitor the fuel quantity; and after the Before Landing checklist (to the line); We decided to leave the #2 engine running on the approach; but due to the possibility of a go around (GA); we briefed an Engine out GA. We were watching for and expected the fuel leak to increase again when the throttles came up on approach. I also instructed the FO; that after landing; with the throttles in idle; I would take the aircraft around 60-80 knots and direct him to shutdown Engine 2 as we slowed to a stop.The FO did a textbook approach and landing. I took the aircraft at 70 knots and he cutoff the number 2 engine.ARFF stated that they only saw drips of fluid near the landing gear and nothing significant. The fuel quantity did not change after we shut down the number 2 engine. We called Ops to get a gate; and ARFF followed us back to the gate uneventfully. The fuel was approximately 6.0 on the left and 4.3 on the right side.* Side note*. We had just flown this aircraft on a ZZZZ turn; and landed in ZZZ with 2 write-ups. (1) Engine 2 oil needed servicing; and (2) LEFT PACK light illumined. We were the ones that ran the shutdown checklist. We left the airplane to go through customs; back through TSA; and returned to the aircraft. We accomplished the preflight and ran the Before Start Checklist. I remember the crossfeed valve being closed. I do not remember it being open at any point prior to the event. Either it was done while we left the airplane to go through customs (and we missed it on the preflight); or I unintentionally bumped or moved it at some point. I do not believe the crossfeed valve was the culprit given the fuel rate continued several minutes after the valve was closed and only slowed/stopped when the engines were at idle.Cause: Possible engine fuel leak."