A Mechanic reports a B737 flew without a window heat controller; because he did not document removing the controller to troubleshoot a left sliding window heat inop write-up; on another B737.

Narrative

I was assigned a log write-up on Aircraft X (Left Sliding Window Heat inoperative). While trying to troubleshoot the fault; I borrowed a window heat controller from Aircraft Y. (That aircraft was undergoing a heavy repair that involved removing the left leading edge horizontal stabilizer panel for repair or replacement.) The Aircraft Y window heat controller was eliminated as the cause and was not used because it was not the source of the fault. I placed it on the floor in the E&E Bay of Aircraft X and continued troubleshooting the cause in the cockpit. I found a broken wire at the canon plug attach point on the P side of the plug to the sliding window assembly. I went and began to reference schematic references and Operations Check Manual references to begin my repair. Accomplished the repair and operations check complete signed off the repair and neglected to clear the E&E area for the troubleshooting part from Aircraft Y. Simultaneously; the Aircraft Y flew without the left window heat controller reinstalled and was left loose in the E&E bay of Aircraft X. I was notified the following day from a Lead Mechanic that a window heat controller was found by an associate mechanic. I then immediately notified my Flight Safety Representative and Immediate Supervisor and started investigating location of aircraft and part availability. I was very interested in providing a good airplane as soon as possible to allow spare availability to our company. I should have taken more precaution in documenting the removal of the window heat controller from Aircraft Y to prevent this from occurring. Documentation of a removed part from any aircraft should be the priority; regardless of time lines and pressure from any source.