It happens, and it is probably more common than the staff would like to admit.
Halfway houses are unique in the reentry landscape because they are one of the only supervised settings where men and women actually share common spaces. In prison or jail, that is simply not a reality. The facilities are either fully segregated or contact between male and female populations is so tightly controlled it is essentially impossible without serious risk, including paying off staff, which does happen but carries consequences that most people are not willing to take on.
At a halfway house, the calculus is different. Yes, the rules are strict. Residents have curfews, movement restrictions, mandatory programming, and staff doing checks. But people who have been incarcerated for any length of time are highly motivated and reasonably creative when it comes to finding privacy. Common areas when staff coverage is thin, brief windows during approved outside time, or simply timing things around the house schedule are all in play.
The risks are real. Getting caught typically means a violation that sends you back to custody, which is about as serious a consequence as exists in that setting. That stops some people. It does not stop everyone.
The broader point is that halfway houses sit at an unusual intersection of restriction and relative freedom, and human nature does not pause because someone is on supervision. Staff at these facilities know it happens. The question is always whether they catch it.