Purification From Madhi

If you feel wetness but when you go to check there is nothing, it is Wasaawis (whispers from Shayṭaan). Wasaawis are doubts without any bases in Sharīʿah. However, if now and again, you feel something and your feeling turns out to be correct when checking later; you should always go and check, if you felt something coming out during salaah. 

If you see later that your clothes are soiled with Madhi which is equal or less than the hollow of your palm (which approximately has a diameter of 2.5 to 3cm) then Salaah in such clothes will be valid but Makrooh-e-Tanzihi (undesirable).  If it came out during Salaah, your wudhu will break as well.

If it is more than the hollow of your palm, then to read Salaah with such clothes will be Makrooh-e-Tahreemi (reprehensible). And one should have to repeat the Salaah that is performed in this impure state. 

And one will need to wash the soiled portion three times before it becomes pure. After the first wash squeeze the water out from your underpants, and wash that part for the second time. In this manner wash the soiled portion of the underpants three times. [Baheshti zewar Pg.75] 

We recommend you to keep using some tissue in your pants. This will absorb any discharge and you will not have to worry about soiling your underpants. However, it would be advisable to remove and change it before performing Salaah even though one is uncertain as to whether Madhi got onto the tissue or not.