It doesn’t make sense as a whole. But, for example, what was he suspicious of?

"I'd slip in suspiciously" means the "slipping in" was suspicious.

You sure? I wasn’t.

“John regarded Mary suspiciously”

“Sharon suspected her husband of cheating. She looked through his emails suspiciously.”

	▲	tczMUFlmoNk 6 hours ago \| parent [-]
		It can mean either. "Suspicious behavior" doesn't mean that the behavior thinks that you've done something wrong. "She's suspicious" can mean either that I suspect her intentions or that she suspects someone else's intentions.