Old fixtures and the gaps around their pipes
Census estimates put Sierra Madre's median year built at 1959, and the city's 59 designated landmarks include a Craftsman bungalow from 1908 and canyon cabins from 1913 to 1922. In houses of that age, pipes have often been repaired or rerouted more than once, and the escutcheon plates, wall cutouts and cabinet backs around them are likely to have gaps.
Those gaps open into warm, damp wall voids where German cockroaches breed out of sight. That is why we place gel bait at pipe penetrations, behind pedestal sinks and vanities, under laundry tubs and near water heaters, then recommend sealing once activity drops. The larger American cockroach is a separate problem: it usually lives in sewer lines and drains and comes up through floor drains, cleanouts or crawl-space openings, so we check those as well.