One thing to bear in mind is the current air temperature. It was -19 last night (Aberdeenshire), so it's not unusual for tubes to go soft relatively quickly.
The Zodiac Pro tubes are made using Strongan Duotex material (PVC). They're also available in hypalon, but more than likely it's PVC.
I don't think the Pro 550 came with pressure release valves, however they may be inter-communicating valves? One thing would be to check the rubber diaphram in the valves. They deteriorate over time and air loss will occur, although the cap should provide airtight seal. A zodiac dealer can order replacement diaphrams. Details here: http://www.westportmarina.com/zodiac/ZodValveParts.html
The tubes are covered by 5-year warranty, so if the seam has separated then it should be covered. A repair can be carried out, but this should be a last resort. Check with main dealer first. You'll need the yellow warranty paperwork in order to claim.