Posting parking lots doesn't mean anything.
This specifically exempts licensees from obeying "keep out, evil gun owner" signs on land:
943.13 trespass
(1m) Whoever does any of the following is subject to a Class B forfeiture
(b) Enters or remains on any land of another after having been notified by the owner or occupant not to enter or remain on the premises.
This paragraph does not apply to a licensee or out-of-state licensee if the owner's or occupant's intent is to prevent the licensee or out-of-state licensee from carrying a firearm on the owner's or occupant's land.
943.13(1m)(c)(2) talks about being on nonresidential property or in a nonresidential building, and specifically exempts parking lots.
While carrying a firearm, enters or remains in any part of a nonresidential building, grounds of a nonresidential building, or land that the actor does not own or occupy after the owner ... has notified the actor not to enter or remain ... T
his subdivision does not apply ... if the firearm is in a vehicle driven or parked in the parking facility, to any part of a building, grounds, or land used as a parking facility.