Cisco isn't sure what Sirius means, if the comment is specifically directed at him, or what sort of promise Sirius is insinuating. What Cisco does know is that Sirius didn't explain it because he wants Cisco to ask. If Cisco was a stronger person he'd just shrug and let Sirius do the work of filling in the blanks, but there's no way Cisco's not going to go crazy if he doesn't find out what Sirius is implying.

So he gives in. "What kind of promise?" he asks, pitching his voice low in a way that he hopes comes across as sort of sultry.