Help identifying an insect (I think it was an insect)
This is not easy to answer without a picture. However, if it was unsegmented, it was definitely not an insect (which have a segmented body plan, both as larvae/nymph and adult). Your description also sounds nothing like an insect. However, the overall description fits fairly well with a flatworm (Phyluym: Plathelminthes), see e.g. Geoplanidae. They can resemble slugs or leeches, but both lack segmentation and antenna. Finding one inside your house sounds a bit strange though, as they generally prefer moist habitats.

Terrestial flatworms are relatively mobile and most are predators or scavengers 

Example pictures: