Is the recurring smell of urine in the underwear natural? And is it obligatory to perform istinja' and sprinkle water when in doubt about urine discharge? And what is the preferred ruling regarding the prostration of forgetfulness (sujud al-sahw) for one who frequently doubts?

One should disregard obsessive thoughts. The presence of a urine smell on clothes does not necessitate washing them and does not invalidate wudu’ unless one is certain of the actual presence of urine, based on the Prophet’s (peace be upon him) saying: "He should not leave until he hears a sound or finds a smell." This hadith establishes the principle that things remain as they are until the contrary is certain. There is no need for istinja’ or sprinkling water on clothes merely due to a urine smell. However, it is recommended to sprinkle water on the private parts and underwear after urinating to prevent obsessive thoughts. If one's doubt becomes excessive, reaching the level of obsessive thoughts, they should not pay attention to it, and no prostration of forgetfulness (sujud al-sahw) is required.