The "no block" flag for SIP2 checkin messages has never been implemented. Bug 32515 fixed the issue where a non-existent subroutine was called if the no-block flag has passed. We should implement a no-block version of the checkin subroutine.
I've been working on this, and I'm not actually convinced we need anything more than what Nick has done on bug 32515. If someone can point out a scenario where we need more please let me know! Kyle