No sucking sound?
Back in 2013, I built my return assembly for the 400g version 2.0, and when I did so I used a check valve installed backwards, which I refer to as the Reversed Check Valve. When the return pump flows, water pushes the valve shut, but as soon as the pump has been powered down (intentionally or unintentionally), the backsiphon of water sucks the check valve open. When this happens I hear a brief sucking sounds letting me know the siphon has broken.
Until last night, that is. I just got back from a multi-city trip. It was time to feed the reef yet I didn't hear the sucking sound. Instead over the next few minutes, I watched several inches of water quietly drain out of my reef, filling the sump significantly. That was new. lol I removed the check valve, and found that the flapper was sealed shut as if it has been glued. Forcing the flapper open with my finger, it was next cleaned out and then reinstalled. I'll keep an eye on it in the meantime, but it may be time to replace this important part that has run flawlessly until now.
Never let your guard down.