also, vader (I believe) is much more powerful than dooku was, so it makes sense that while vader was occupied, he would still be able to sense obi wan...especially since he hadn't sensed any jedi i many many years, seeing as how they were all dead. sensing a jedi in the days of the old republic would have been a bit more common and because of that, it may have slipped dooku's attention. then the added fact that he was preoccupied planning a rebellion would make it harder
The Jedi have all kinds of skills to cloak there presence, like pulling the Force in, he could have even been blocking Dooku's senses. With Dooku thinking the Jedi wouldn't find out about his whereabouts so quickly, he might not have been even remotely tuned into his surroundings (again, Sith overconfidence). Also, you don't just sense something if you're not even remotely looking for it, on top of that Dooku had never even met Kenobi.
"Its a shame our paths have not crossed before. Qui-gon always spoke very highly of you".
So his energy signature in the Force is foreign to Dooku, and Obi-wan was obviously cloaking his presence. Its apparently one of his strong points as he meandered (almost in plain sight) through the Death Star in ANH without any trouble, even making the Stormtroopers hear that noise so that he could get away from the tractor beam generator.
Vader sensed Obi-wan because he has an extremely strong bond with Obi-wan. His energy signature was well known to his senses. And he's the Chosen One 😄 which excuses any of his odd behavior (sarcasm).
As far as Maul is concerned, I think he COULD sense Obi-wan was about to do SOMETHING, he just didn't know what. Watch the scene closely and you'll see a shot looking up at Maul where he's watching Obi-wan and his brow furrows suddenly like "what are you doing...?" apparently sensing a tremor or manipulation in the surrounding force. Then Obi-wan springs into action. I don't think Maul knew he was capable of doing THAT to him. Man that scene was cool as hell though!
i said that- they have a bond, they can probably sense a whole lot of things and make a habit of "turning down the volume" so they don't go nuts. But when you trip over a persons energy you've felt before you listen to it. Like say you're somewhere where every person has there own boom box and is playing there own song. each person has a different song. well, its easy to tune out a bunch of stuff you don't know, but then you hear a few notes (wouldn't even have to be a lot) from say, oh...I don't know, the Imperial March for example 😄 [enter favorite song] and you'd go hey I know that, and your ears would focus in on that one song. The Force is another sense and would work in a similar fashion.
Re: Darth Maul's fatal mistake?
heres the deal. darth maul could have taken them both out. he took out quigon because he saw him as a greater threat. he could have killed obiwan too,but his fatal mistake cost him his life. mauls death was caused by arrogance. maul didnt see obiwan as an opponent worth taking seriously. therefore obiwan saw that he was being underestimated and used that to his advantage.