My best guess is that it would simply be like with horses in reality. You either have them with you at the time or you don't. Summoning them randomly at any time in-game is just that, a game mechanic for convenience. So they are, as the others have already mentioned, tied up, docked or simply nearby awaiting instruction.
Characters: Andre Winter (Hy'ur) / K'nahli Yohko (Miqo'te)