It's that whole "bodily likeness" part that gets people so excited. Everyone wants to see one, not realizing there are others ways they can be experienced. Yes, I know this isn't very scientific, but they can be a sensory experience. The thing is to consider these experiences in addition to your data.
Here are my top 5:
- Touch: Yes, some folks have reported being touched. It doesn't seem to be an uncommon experience. I know of times where it has felt like the touch of a solid person, and I also know of it feeling as if you've stepped into static: all prickles and tingles around you.
- Smell: One reason not to wear fragranced products while on investigation is that sometimes they--ghosts--can be smelled. Cigar smoke, flowers, perfume, gunpowder--all of these things have been reported before.
- Feel: Whether by temperature (sudden temperature drop, or the opposite--warmth), or that funny "thick" feeling some people get where they can become woozy or short of breath.
- Hearing: Footsteps, voices, and other unusual sounds that are not being naturally caused.
- Sight: Shadows, balls of light, sudden "mists" that are not fog, full or partial apparitions
Apparitions are rare things. You're more likely to have 1-4, or variations of 5 happen instead. If you go into this hoping for your stereotypical ghost, you're going to be one disappointed person.