Hi Mark,
Risking it will come back to bite you some day............hopefully not anytime soon.
The reason the plec may be hiding is if he thinks the Altums are large enough to be predatory he might behave with safety in mind and might only come out at night or after lights out. Catching him and putting him in a net will only cause unnecessary stress IMO. Therefore I think it is better to turn off lights early (before you go to bed) put some food in the tank and see if it tempts him out (with the use of a torch).
If you have an open plan tank then that could also why he feels unsafe. You could pick up a few slate caves (or terracotta) and you should be able to get them easily enough on ebay or from a locally catfish club.
IMO plecs can be fussy feeders so he will not feed unless he is accustomed to the food presented or loves it. Try and feed him what he wants and hope for the best

If you need further advice then I can recommend as a great place to learn about plecs all shapes and sizes.
HTH.
Best Regards...............Tony