The consonant /l/ in English is usually realised as an approximant. This means that although the tongue in such instances makes contact with the alveolar ridge, this doesn't prevent the air from flowing freely out of the mouth and there is no increase in intraoral pressure. The resulting sound is therefore phonetically vowel-like. The only difference between the production of an [l] and a vowel is that in the production of an [l] the blade of the tongue makes contact with the alveolar ridge and the side rims of the tongue are lowered and thus the air doesn't leave directly through the front of the mouth but flows out of the large side apertures this creates.
The resulting sound has an underlying vocalic resonance, or vowel-like quality, which, like other vowels, is affected by which part of the body of the tongue is raised and by the distance between this part of the tongue and the roof of the mouth. In other words, it may be described like regular vowels in terms of its closeness or openness along one dimension and its frontness or backness along another.
In most standard Englishes there are two common allophones of /l/. Broadly generalising, the /l/ which occurs at the beginning of syllables is realised with the vocalic resonance of a mid central vowel, something like [ə]. This is usually referred to as 'clear l' and is transcribed just as [l]. The /l/ that occurs at the end of syllables, in syllable codas, has the vocalic resonance of a close-mid back vowel or similar ( it may be anything from [ʊ] to [o]). This is /l/ is traditionally referred to as 'dark l' and is usually transcribed [ɫ].
The original speaker has noticed, then, that dark l has the vocalic resonance of a close-mid back vowel. They could have a future career as a phonetician!
Vocalic /l/
It might also be that the Original Poster has been hearing instances of vocalic /l/.
For an increasingly large number of speakers syllable-final /l/ is now realised as a phonetic vowel, [ʊ], referred to as 'vocalic l'. In fact, many traditional RP speakers have long been using a vocalic l after bilabials in words such as people, but are simply unaware of it (and many will vociferously deny it right after doing so). The rise of vocalic l in RP, therefore, might be viewed as the spread of vocalic l to environments where it wasn't previously used.