It's not the appearing cups that should concern you, it's the disappearing teaspoons.
The case of the disappearing teaspoons: longitudinal cohort study of the displacement of teaspoons in an Australian research institute
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1322240/
I have observed this when I was a teacher in a High School
Teaspoons were a rare commodity - no matter how many were in the staff room drawer at the start of term after a few weeks we were reduced to stirring with a fork or knife
neither forks nor knives tended to dissappear
Mugs had the same problem - teachers brought their own personal mugs in but they didn;t tend to remain
one bloke was a local fell runner - quite well know locally - and he brought one in which was branded with the logo of his fell running club - and so was easily identifiable - still went missing
I took a really nice Wallace and Gromit mug in - strangely it lasted a few months but then went and a different W&G mug appeared
Every now and again Heads of Department were instructed to check all the cupboards and shelves in their department and return any mugs and spoons - which were then displayed in a big box so people could retreive their property
exactly why professional people with responsibily for the education and morals of youngs people chose to take otehr people's property was never investigated
However, back to the point, teaspoons were never recovered in any volume
at one point I bought about 20 that I saw cheap in a market
after about 3 weeks there were none left - that is a rate of over 1 a day!!