This just means you haven't practiced enough to find it by feelVery true! And is also why lined necks are very popular with many fretless players who regularly gig with one -despite all the purists out there who insist on plain necks. Also, if you move around a lot while you play thise frets or lines can be a godsend at times.
The markers definitely help, but I've been working hard to wean myself off them; the more I practice, the easier it gets.