I was notified by my supervisor that an aircraft was coming in from ZZZ with a broken tamper seal on the forward lavatory O2 mask door. Our initial thought was that we needed to replace the broken seal. We later discovered that a seal is not required and that the tsa in ZZZ had put it on. I questioned maintenance control and they were aware of it and allowed the plane to fly with this seal in place. We took a lengthy delay investigating this and in the end we removed the seal; opened the O2 door and verified that the masks were packed correctly and re-closed the door. Today; another aircraft had the same issue; however; this time we did not know about it ahead of time. Apparently a fleet service supervisor told one of our technicians about the tape this time and I was notified just before push time. This time the seal was not broken so the forward lavatory O2 mask door was disabled by the tsa.
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I can't believe the airline or the FAA would allow the tsa to perform undocumented maintenance on one of our aircrafts; resulting in a disabled emergency O2 system.