The idea of your acolytes stumbling over an old, abandoned imperial post (seems very Jurassic Park to me….) and having to defend their room from prowling nasties while trying to fix a communications device should lead to a memorable session.
There could be many reasons for people to be close enough to pick up their distress signal. The beast house is an obvious example but a big game hunting party is just as likely. Smugglers or a crime gang coming to pick up drug ingredients. A mining team investigating an anomaly on their scanners, hinting at a new find. A voiud captain press-ganging people to top up his depleted crew.
Or you could go a bit further and put in a secret lab by a magos biologos studying the medicinal values of the local plant life. Or take it up a notch; a secret lab in which captured tyranids are being exposed to a variety of bio-weapons (made from local stuff). And conveniently get loose once the acolytes have been taken in and think they are rescued….
Or have them encounter a secret watchtower from the Deathwatch. Guarded by automated defences and manned by servitors and chapter serfs (perhaps with a single battle brother). Created at the dawn of the calixus sector, it was tasked with overseeing the ruins of a defeated xenos civilization on Dusk. Now the tower scanners indicate new movement in the nearby overgrown ruins. Perhaps the acolytes can have a look?