Does this constitute a legal search?

[Macy27]

I'm living at a hotel with my boyfriend and last night we were smoking weed in the truck and forgot the bag in the truck when we went inside. Someone saw it and called the police. The police made my boyfriend unlock his truck so they could search it. They said they could search it without a warrant because the drugs were in plain sight. They found more drugs and arrested my boyfriend. Was this a legal search? I thought a warrantless search was only legal if the vehicle was moving but it was in the parking lot and we were inside. Any help would be appreciated.