Do you ever find fish in weird places? Like they migrate between tanks when you're not looking?
This Ancistrus sp. L509 was a juvie from a group that I moved out back at CatCon 2022 (more than a year!). He just turned up in our C. saulosi tank. No idea how he got there.
May have been hiding inside a sponge filter and you moved it to another tank. Happened to me before with some Loaches.
It is happening more and more frequently in my tanks. The most plausible explanation is that the fish have perfected teleportation.