I am just not certain if there's a reliable way to take enchanted weapon back from someone who stole/took it for whatever reason.
One thing is when weapon drops from character's hands. Another thing is when it's stored and put into inventory.
For example: your character that lost weapon eventually dealt with thief. Thief is captured, but for some reason refuses to hand it over. You'd still need a DM to be present during such encounter to take weapon from thief's inventory. Another case is when thief will be corpsed. Someone can find thief's body, revive them and you'll have to repeat capturing procedure.
Common sense tells me that cases I briefly mentioned will be a rule breaking, however I don't see any line about how encounters with stolen enchanted weapons should be handled.
The way I see situation, if there's absolute consent between all players involved current system will work fine. The moment someone would want to be overly mean towards another DM involvement can't be avoided.
To be clear, if a thief takes something of yours through breaking the rules (e.g. pickpocket without hostiling first), that's cheating, and you should take screenshots to prove what happened and what you lost. In that case, even if the thief can't be found because they logged off permanently or whatever, a DM should be able to restore your weapon for you.
If a thief takes something of yours through legitimate means (e.g. picks it up after you're killed, pickpockets you while they've hostiled you), then while it's perfectly okay to portray this IC as theft, from an OOC rules perspective, it's technically their gear now, and a DM is under no obligation to intervene.
Enchanted gear is no different, as per OOC rules. Because enchanted gear is both highly valuable and useful to its original owner, and useless except as a trophy to everyone else, there's been a bit of an OOC culture of respecting it more; but in fact if they take your enchanted gear through rules-wise legitimate means, it's now theirs.
Some time ago I've heard that according to rules you can't demand from another player to drop any item from their inventory for you. Is that correct?
That's correct.