Hi, this is common.

If you even suspect he's using that method, just kick him.

Reasons:

1. It's not polite to do that to any group without asking, I always make sure I give them consent before letting anyone into my group now since they come to take support.

2. He could also be looking out to see if anyone is convoying SS/plundering at the moment. Your members don't deserve to get plundered by a spy imo.

I have many low levelled/low bp people who just stand at my group's convoy NPC, and I give all my anbu/seniors permission to kick anyone who does that, because it's not safe for them. I do allow people to bring in their alts in my group if they ask nicely and monitor them to check if they're spying.

Is he being continually plundered by the same person?

If yes, take this up to your leader and tell him you suspect it. I hope your leader is willing to listen.

If no, but it's generally people from the same group, all the same take it up with your leader.

Especially if that account has taken strong support and still loses.