Your on the right track in wanting to find the source. Consider this, phosphates come from food and waste. You are not over feeding by any means and you are not overstocked. So this means you have waste sitting/trapped somewhere in your system. Heres a few places it could be hiding:
Sand bed (not just on the surface)
Filter floss or media
Dirty equipment (food clogged powerheads, detritus riddled sump, skimmer, refugium etc.
29g Mixed Reef, 150w 14k Phoenix MH w/ Moonlights, Reef Octopus BH1000 HOB Skimmer, Ecotech Vortech MP10es, ReefKeeper Lite (Plus), PH Probe, Temp Probe, ATO, Carbon/GFO Reactor.