Turnstiles aren't theater and Redis doesn't make password storage secure so the entire thing seems a little el-el-emish..

But what about that sentence does that not make sense? They are describing tailgating..

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.”

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.