4-4 point low approach low extension, 3-2 contact play on the slide
is an example of a non-joseki continuation of this standard variation. This idea occurs also when is played at a. The joseki play is at b. Black's tenuki is possible, but the contact play is hardly ever justified
Generally it isn't good for Black to give up the corner this way. White lives quickly - the ponnuki gives good shape in sente, and White can now turn to the outside.
For Black to play this way instead is just about imaginable in special circumstances. White presumably connects the ko immediately with , for a good local result. Black would feel that the marked stone was inefficient, normally. If Black urgently needed to defend, taking sente and then playing a might possible make sense.