Fourth Amendment.
As someone might have been hiding in a hollowed-out mattress, gun found under mattress during protective sweep was properly admitted.
http://www.ca5.uscourts.gov/opinions/pub/14/14-41392-CR0.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.