^That sounds like a classic blow out from having the tube under the rim. Blows out loudly and violently with a big hole in the tube, and often no marks on the tire.

Admittedly that the tire was on the bike for 3 weeks makes it a bit unusual, but it could have been that the tube was pinched under the bead, and took a while to work out to the point that the tube expanded outside the tire and blew out.