Is there an industry term for IC "drop-in" circuits used for malicious or clandestine purposes?
Trojan is the term. Specifically Hardware Trojan. This term is generic as it applies to any hardware that produces this unwanted security hole, including MITM hardware, equipment with circuit boards replaced, firmware replacement, add on hardware like mod-chips not disclosed to the end user etc. Not specifically just wafer level changes to an IC, but that's included.

See https://www.sbir.gov/sbirsearch/detail/1319177 for the Design Tools for Hardware Trojan Detection and Mitigation article provided to the Department of Defense.