That is
not an 'urban legend'.
Hidden Mickey's are (or were?) not official or sanctioned, and were put in by the folks who actually built or painted or decorated the attractions as 'inside jokes'. Some are simply an arrangement or positioning of items to create an image or shadow that creates the three circle image. An example is certain tables and chairs near Space Mountain that will be sometimes arranged by a CM to cast a HM shadow in the high sun.
The concrete 'battle' I was told of took place in the dining room of the Haunted Mansion, where each night one of the cleaners would arrange a place setting to create an HM with a dinner plate and two smaller plates.
In the morning, the inspecting Imaginear would carefully rearrange the setting to where it 'should' be.
This was countered at one point with the entire table's place settings being arranged to create HM's.
This was countered with glue.
Mediation took place, and it was agreed that one setting would be allowed to be an HM and nothing else altered from the way the Imaginears had it set.
Thus, if you look very carefully, you will find one- and only one- HM on the table in the dining room at the Haunted Mansion.
Jud