They suspect they'll hear him before they see him - assuming they need to. Despite knowing what they do, seeing what they've seen, it seems...they really don't know very much about what he's been living through in the recent months, have they? That fear of overstepping lines, of venturing where they're not wanted, afraid to push him ever deeper into the embrace of someone they drove him to the first time.
They ruined it, the first time. They know they did.
FRISK ; CLOSED to METTATON ; we won't go, we don't know when to QUIT
They ruined it, the first time. They know they did.
The least they can do is try and make it right.
They try his room, first.