"The actors are not in the building."
Translation:Histriones non sunt in aedificio.
I agree. Esp. since there is an expressed subject, in a simple sentence the verb 'to be' could be absent. Harm Pinkster (The Oxford Latin Syntax. Volume 1: The Simple Clause [Oxford: Clarendon, 2015] 201-204) states (204) that all forms of sum "seem to be omissible, with the exception of existential sum." He covers existential sum with examples on p. 200.