Ghost, your experience is a hard & fast example of how false info can be forthcoming from public officials, and/or the "no one cared till you asked" pyschology. And even though YOU questioned it here on this forum (thankyou), yet you'd be surprised how often info like this ("such and such city is off-limits" or "such & such entity's land is off-limits", blah blah) gets simply taken for granted! Then spread on-line (like in threads, or on lists of rules for various entities, etc...), and before long, it's just taken as fact!
I realize that was just a librarian telling you that, but believe it or not, such answers can come from city people (park's dept, police station front desk, etc...). They too can tell an md'r "md'ing is not allowed in the parks", when, in fact, it's not really written anywhere specifically saying such a thing. So the information gets put in the rumor mill (afterall, you wouldn't want your friends to "get in trouble"). And pretty soon, it's just taken as fact.
Your story is the perfect example of how you can get "no's", when there really is nothing saying that type thing. Thus better for us md'rs to look up rules for ourselves (if you're skittish). If nothing there saying "no metal detectors", then presto, it's not prohibited.