ZOB ARTCC Controller reported during flight breakup exiting refuel track one of the aircraft took evasive action to miss a low wing aircraft not in contact with ATC.

Narrative

I was training. About 10 minutes into my session; Aircraft X and Aircraft Y were ready to break up along the R193 refueling track around REC VOR. Aircraft X advised me that Aircraft Y will be requesting a clearance back to MDT at 090; I told Aircraft X to have Aircraft Y squawk a code and contact me on my frequency. Aircraft Y soon checked in with me level at 130. Soon after; I radar identified Aircraft Y and gave them clearance direct MDT and to maintain 130; I then gave Aircraft X his clearance to MTC and terminated MARSA. Aircraft Y then comes back to me and says they are taking evasive action to avoid traffic. It was at this point where I noticed a mode C intruder target indicating 130 on a northerly track merged with Aircraft Y. Aircraft Y made a right turn and descended about 500 feet to avoid the intruder. It was later discovered that the mode C intruder was possibly Aircraft X; who was performing survey work on a north/south grid pattern in that area for a few hours in which we were never in contact with the entire time. Survey work aircraft need to advise the controlling center ahead of time of their intentions and is highly recommended that they contact ATC for flight following so events such as this can be avoided; especially when aerial refueling is being conducted in the same area/altitude.