After performing a visual walk around of a B747-400; Mechanic is informed the next station found speed tape covering a damaged ground conditioned air connection door panel. No maintenance record of speed tape or damage.

Narrative

Assigned walkaround as part of 3 service before going on contract assignment with another air carrier. Completed aircraft visual; found nothing damaged; signed off item on 3 service. Upon returning to work the following day; my Supervisor asked me if I had done the walkaround on Aircraft X the previous day. I said I had; and that it was a good aircraft. The Supervisor told me that the next station had found speed tape on the aircraft upon arrival in ZZZZ. I told the Supervisor I found no tape on my walkaround. Another Supervisor requested for a statement in regards to my walkaround on Aircraft X. I gave him a statement without a union representative present. I was given disciplinary action by my Supervisor. I am aware that Aircraft X was towed at about XA30 to the gate and remained there until departure at about XD40PM. This is about 5 1/2 - 6 hours. Aircraft external air conditioning was connected and the said taped or damaged panel had to be opened. If panel was damaged; Ramp; Gate Captain; flight crew; made no mention of speed tape on the panel. I suspect damage was done either at ZZZ just before departure or upon arrival at ZZZZ; whoever placed tape on panel did not report it to Maintenance at ZZZ. Again no tape was visible on my walkaround at XU30.