If a knight goes missing from his lands (Fumbles a passion roll, is held captive in the Faerie realm etc) what happens next? Does his heir get knighted immediately in the same way he would if the older knight died or does a certain amount of time have to pass first? What happens if the missing presumed dead knight returns many years later? Does he just assume control of his lands again, with the heir stepping down (but still a knight)?

I'd say it depends on the situation. If the Liege needs knights and the heir is of age and able to take charge of the manor, then it might happen more quickly. If there is peace on the land and/or the heir is not able or of age, then it might take longer. Does the Liege like the heir better than the previous owner?

Generally, I'd imagine that there would be a period of grace, like a year. But after that, if the knight is missing, he is not holding up his end of the feudal bargain, so...

In my campaign, I decided that after seven years of absence, the knight would be declared dead by his lord, and then inheritance would be solved as usual.

If a knight goes missing from his lands (Fumbles a passion roll, is held captive in the Faerie realm etc) what happens next? Does his heir get knighted immediately in the same way he would if the older knight died or does a certain amount of time have to pass first? What happens if the missing presumed dead knight returns many years later? Does he just assume control of his lands again, with the heir stepping down (but still a knight)?

I too think that seven years absence = death. If the heir is of age he inherits.
If the heir is a child the lord takes the kid and his mother into his hall and runs the manor.
When the child is of age he gets knighted and inherits
if the lord obeys the rules anyway
which might be therein a story

And the old man comes home after eight or twenty eight years?
Therein IS a story
no precedent exists for such an act!
(that I know of) :)

I've got the interesting case that one of the player knights has been missing four years, has no heirs, but the players are about to go off and find him (he's been kidnapped by faeries), but, as we're entering the Anarchy period and all the nobility are dead, I imagine the countess would be very happy to have a knight come back, even if it's a bit irregular.

I guess she would, but it all depends on the story he is going to tell and what he does to get back in her good graces. ;)
Also would she let the others go? There are saxon on her doorstep and no lord. she needs them at home, not running around searching for a friend that has gone missing for four years. It could make a nice conflict of interests for them (loyalty vs the countess against their loyalty for their friend).

Well, the guy who went missing has a vassal or two as well, and will be in his 30s, so one of the few "old" knights left, I was thinking that the countess would want him back if possible, but that's a good point that she probably wouldn't like 4 of her senior knights (one of him is technically a banneret now) going to the Forest Sauvage to see if they can find him.

I've made vague hints that they might be able to get faerie help to protect themselves from the Saxons, so maybe the Pagan in the party will try and convince the countess with that argument, so she might be less worried about them going off briefly, though I don't think there's such a thing as a brief sojourn to the Forest Sauvage.

I don't think my players want to get into a high politics game so this dilemna might be a good way to get any knights around their age to take on the boring administration/rulership stuff while they get to go off adventuring and murdering for the Countess.

Would it be possible for them to hire a few mercenary knights to hold their place while they're away? Or perhaps they can just give the money to the countess as a form of scutage?
If they haven't go the money of course, sending them on a quest to find (or take) a few librum could be an adventure in and of itself.

Well, in my case, the player had hired mercenary sergeants to guard his manor, and one of his vassals (who is also a cousin) represented him at court and basically donated everything they could to cover the costs of building the walls around Sarum, so that resulted in the Countess being much more happy to let the other PKs go off and rescue him from the Forest Sauvage.