Firstly let’s note that the abduction of Devi Sita by Ravana was informed to Shri Rama by a dying Jatayu, much before the meeting with Hanuman:
Oh, boy Rama, blest is your eternality, for whom you are searching as with the search of a rejuvenescent herb in the vast of forest, Ravana stole that lady and my lives, too
- VR 3.67.15
Though he was asked by Shri Rama to elucidate details about the Rakshasa, Jatayu was able to tell him only about his illusionary powers and while providing more details he passed away. (Refer VR 3.68.9 and VR 3.68.16-17)
Thereafter, the two brothers meet Kabandha rakshasa and while informing him about Sita’s abduction, request his help as they do not know anything besides the name of Ravana (being the only thing given to them by Jatayu):
Ravana conveniently stole my illustrious wife when myself and my brother exited from Janasthaana. I only know his name but not the form of that demon... and we are unaware either of his stronghold or of his staying power.
- VR 3.71.21b - 23a
Kabandha too didn’t have any other information about Ravana and merely advised them to befriend Sugriva and so they move on in the same way without any information, to Pampa.
There when Shri Rama and Lakshmana meet Hanumanji, Lakshmana informs him as stated in the question:
his wife is abducted by a guise changing demon in a lonely place, of whom we know not... we know not which demon stole his wife...
Here they do share all details with Hanumanji, it is merely not elaborated in so many words. What they mean by the above line matches correctly with the earlier quoted episode with Jatayu and Kabandha about knowing only his name. Here Lakshmana means - we know not anything about the demon that stole his wife (except his name) i.e. we do not know who he is or which demon he is, except for his name.
For further satisfaction as to the fact that not every detail has not been elaborated in this conversion but was duly conveyed (without hiding any facts) is that Sugriva tells Shri Rama that Hanuman also informed him about Jatayu’s fate:
And he also said about that arch demon's killing an eagle named Jatayu on waiting for an opportunity, and his causing an agony in you by way of separating your wife.
- VR 4.6.4
Now Lakshmana telling Hanumanji about Jatayu doesn’t find mention in that conversation. Nor does the telling about Jatayu by Hanumanji to Sugriva find mention anywhere in Sarga 5. Sugriva merely tells Rama - yes Hanuman told me.
The above conversation must be merely interpreted as Lakshmana told all details to Hanumanji.
There was no concealing of any facts but the same have not been elaborated.