ian anderson - I'm not familiar with your setup so parden me if I'm being repetative or anything...
When animals and plants respirate / breath, they release carbon dioxide - which reacts to all the calcium-based sand and LR in our tanks. The CO2 and the calcium react and in the process help stabalize/raise the PH. Once the lights go off however, animals slow their breathing and everything that releases carbon dioxide, releases less than during the day. Therefore there is less reaction between CO2 and the calcium, and therefore less PH-buffering. If you test the PH after the lights have been off for couple of hours, or first thing in the morning before the lights have come back on, you will always find the PH to be a little lower than during the day. This is the reason many people run their refugiums either 24hrs/day, or on a reverse-schedule to their primary lights. Now, you mentioned testing in the afternoon and it was 7.6... Assuming the lights have been on for several hours at this point, I would have expected the PH to have come back up from its lights-off, evening dip.
Any chance you have Ammonia in the system? The presence of Ammonia drives down the PH......although probably very unlikely to randomly have Ammonia in your tank unless something died overnight...
Ditto the PH buffers....good stuff.