I have been watching a series of "Great Courses" lectures on the Vikings. In it the lecture they mentioned that a group of lost Rus Vikings petitioned the Byzantine emperor to help them get home. It mentioned that Louis the Pious, after questioning them, had them imprisoned until he could "figure out who they were". A little internet research found that they were imprisoned after Louis the Pious realized they were Swedes and suspected they were spies. Neither said anything about them ever being released.
So... were they ever released? It seems incredibly heartbreaking for a group to travel from Sweden, cross the lands of half a dozen or more different peoples, sail down to Constantinople on the Black Sea, petition an emperor of a foreign state to help them find their homeland, follow this delegation all the way to the H.R.E., and then be imprisoned forever because they think you're a Viking spy, only a (comparative) hop, skip and a jump away from home.
I took a look into this and found an account of their temporary detention in the Annales Bertiniani for the reasons your indicated. It does however state that they would be freed one way or another once the matter was concluded. Roughly translated it says that if they would be determined to be faithful or genuine [si fideles invenirentur] they would be given the capacity to return home without danger [et facultas absque illorum periculo in patriam remeandi daretur] and would be given aid to do so. If however there were doubts about their identities and purpose they would be returned to Theophilus so that he could make the decision himself [ipse decerendo efficeret]. Annales Bertiniani, ed. Waltz, G., SRG (Hannover, 1883), 20.
All of this is to say that one way or another the text indicates that they would be leaving Ingelheim where they were detained. The question is how long it would take and by what route. It does not seem that their lives were in danger from Louis as he had pledged himself to safeguard them while in his custody, and the warm references they carried from Theophilus lead one to believe that they had little to fear from him either.
I will do some more digging and see if I can find any other references but take heart, it does not appear that they met their end in Germany.