Relying on another Mechanic to verify the internal tail section of a Q400 aircraft was clear; Mechanic X reports he closed the Tail access panel and signed-off a CVR Underwater Locator Beacon (ULB) Check. Approximately three weeks later; Mechanic X was notified the Tester equipment he used for the ULB Check was found in the tail of a Q400.

Narrative

I performed a CVR Underwater Locator Beacon (ULB) Test in the tail of a Q400; Aircraft X. When completed; I gathered tools and removed them from area. I then asked Mechanic Y working on the APU; to verify that I had left nothing in the tail. He said that it was clear and I closed the access panel. I returned to Avionics area and had to search for the proper location for the ULB Tester (I did not remove it from the Tool Drawer area; it was sitting on the bench when I arrived on-Shift. Mechanic Z was preparing for the task and then had me do it while he set up for the Transponder Functional Test). I didn't see the proper location [for the Tester]; so I placed it in the second drawer from the bottom where I thought I had seen it months before. Approximately two weeks later; the Dayshift Lead asked us if we knew where the ULB Tester was at. I went to the drawer where I had placed it and it wasn't there. The Lead and I searched the Tool Drawer area and didn't find it. The other Avionics guys said it was probably in ZZZ1 because it; along with other equipment; was always being sent there. Later that weekend; I asked Parts Employee X; in Stores; if they had it by mistake and he thought it had been pulled to turn in. He said he would check the [Station] authorization list to see which one we had; but he thought that we weren't assigned one [Tester]. I didn't hear the results. Five days later; a Mechanic told me that a ULB Tester had been found in the tail of Aircraft Y; another Q400. I asked the Supervisor about it and the paperwork showed that I had been the one signing for doing the test on the ULB. (At that point I didn't remember which plane I had tested; it was only the second time I had done that test; the first being many months before). I told the Supervisor what I had been doing that night and that I had turned the Tester into the Tool Drawer area. It [the event] stood out because I felt stupid for not being able to remember or find the right place to put it in the Tool Drawer; and I remembered asking for the second set of eyes. A Mechanic found the Tester in the tail during an A-Check. I am new to the work group and am still learning where everything goes and the procedures of the group. I didn't verify that the Tester; already sitting out; was signed out and I didn't look for the tag on turn in. I took for granted that whatever had to be done prior was done. Suggest adding a visible Sign-out [Sheet] that has to be confirmed by another Technician when a tool is turned in; either at the time or at [shift] turnover.