What is the cause of wetness on the penis upon waking, and does it necessitate ghusl, given that no nocturnal emission is recalled, nor is there any trace of semen or pre-ejaculatory fluid?

If the wetness is merely an illusion, then do not pay attention to it. However, if you are certain of the presence of wetness on the male organ, then if you find in it the characteristics of semen, a ritual bath (ghusl (definition)) is obligatory, even if you do not recall a wet dream. If you find in it the characteristics of prostatic fluid (madhy), then ablution (wudu) is obligatory, and you must wash whatever parts of the body and clothing the prostatic fluid has touched. If you are uncertain whether it is semen or something else, then choose and assign to it the ruling of one of the doubtful possibilities.