Indeed. They should not have got involved in a 'misdemeanor' and compromised themselves. Any potential hijacker could use such a technique to identify and neutralise marshalls. And the crew should not know who they are either - that could lead to a potential compromise. However, the times I've flown on US flights, the marsahlls couldn't have been more obvious.
When some UK airlines carried them after 9/11, the crews were just told that they may be on board. There was nothing to identify them, and we would not know if our flight was designated a marshalled flight. I relaised I might have one on board once when someone I recognised from the military was sitting in economy. We made eye contact but showed no sign of recognition.