"I'd slip in suspiciously" means the "slipping in" was suspicious.
replyYou sure? I wasn’t.
reply“John regarded Mary suspiciously”
“Sharon suspected her husband of cheating. She looked through his emails suspiciously.”
It can mean either. "Suspicious behavior" doesn't mean that the behavior thinks that you've done something wrong.
reply"She's suspicious" can mean either that I suspect her intentions or that she suspects someone else's intentions.