Can I sue over a chargeback if the guest never signed anything at check-in?

A missing signature narrows your options without closing them. California recognises claims on a running account of charges between two parties, which is close to a description of what a folio is: dated line items, a balance, and a course of dealing. Where you have no signed authorisation, that account becomes the spine of the claim, and everything else you bring is there to establish that the account reflects a real stay by a real person. The reservation record ties the name to the booking. The confirmation email ties the name to an address you can serve. Door lock or property system logs establish that somebody with that key was in that room on those nights.

Practically, the absence of a signature shifts what a judge is testing. With a signed card, the question is whether the terms were breached. Without one, the question is whether the guest occupied the room and understood they would be billed for it, which is not a demanding standard when the reservation and the access logs line up. What genuinely hurts is a gap in the middle: a booking with no arrival record, or an access log that shows nobody entered. If your property does not retain lock logs, changing that retention setting is the highest-value operational fix available to you.