Debí dormirme andando
 

Here is another phrase from Tintin comic book.

CONTEXT: The Captain has hit a tree and passed out and then awakens.

PHRASE: Debi dormirme andando.

"Debí" is introducing the hypothesis about why he's just waking up when the last thing he remembers was to have been walking.

Although I'd prefer to say "Debí haberme dormido andando"

Cabe destacar que puede distinguirse entre "deber" y "deber de". Aunque se admiten ambas formas para hipótesis, con la preposición se anula cualquier ambigüedad:

Debí de haberme dormido andando.

Un saludo cordial.

Okay, thanks, still a bit confused, I'll work on it.

If you can translate into an English sentence, that might be easier for me,
but maybe there is no literal translation.

There is a close translation. Try a sentence starting by "I must have..."