Hopefully and most likely an airlock however it could be an issue with the heater matrix or the bypass valve.
Has the coolant level dropped in either the expansion or overflow tank if so that would point to air coming out of the heater matrix which is now trapped in the 'wrong' place could cause overheating. You could try running the car for a bit with the heater on max and the expansion tank cap off to see if you get any bubbles - also try manually 'burping' the top hose with the cap off as well (open when cold!). If there's no joy with that and it keeps happening the next step would be a coolant drop and I'd also consider flushing the heater matrix with fresh water to see what kind of state it's in.
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2005 Spec B 6MT - KiDO tuned (~270Bhp)
Custom exhaust - Cosworth filter - Samco Intake - Tints - Bilstein B8/B6 & STI springs - SuperPro ALK - Whiteline positive shift kit, rack mounts, rear adjustable camber bushes, front ARB mounts & strut brace, 20mm rear ARB with AVO mounts, - Hardrace Gearbox and pitch mounts - Prodrive PFF7's - Front Brembos - Kenwood CarPlay - 1/2 carbon grille - STI front splitter
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