Probably nowhere. Picnic baskets that are formed by spawners on certain community lots are meant to be enjoyed on-site by visitors to those lots, not stolen and placed into an inventory. I know that's not realistic and one might have to question if proper food handling procedures are followed in preparation of the goodies inside those baskets, but I've never seen a sim get sick by eating from them.

Do your sims need directions to the nearest grocery store, or maybe into Build/Buy for proper objects that can be owned?

If you want the baskets to stop appearing, and I wouldn't blame you if you find them annoying, you could go into MC > Object Stats > Non-Inventory on the lot in question and select the picnic basket spawners (if you can find them on the list that will appear) to flush them away.
(MC Cheats add-on module required)

There are also several picnic basket mods out there, but I've lost track of which ones are supposed to do what at this point.

If there are no such spawners on the lot, then I would have to say it's the community lot picnic benches that are producing the picnic baskets. A different approach then would be to remove them and/or replace them with residential lot picnic benches (Buzzler's Build/Buy Restriction Choker mod probably required). It may not be worth the trouble though if the baskets are now working properly unless you still find their presence annoying.