Collins dictionary says that "panneau" is pronounced [pano] and "art" [aʁ]. However, the [a] in "panneau" does not sound like the [a] in art, but rather more nasal. Am I listening that right? If so, why is the nasal sound of "panneau" not represented in the phonemic/phonetic transcription provided by French dictionaries?
The widespread pronunciation is [pano] with no nasalisation. You might hear [panɔ] in eastern France, but this is unrelated to your question.
A non native speaker might hear a slight kind of nasalization at the end of the vowel a due to the transition to a nasal consonant. This is probably what you experienced but native French ears cannot hear it, or at least won't assimilate it to a nasalization.
There is an exception about this lack of nasalization but it doesn't really apply to panneau. It is the pronunciation used by a few people, generally old, from the Occitanie region (e.g. Toulouse, Montpellier) who might still nasalize the sequence an or am before an n or a m leading to [ɑ̃n] (or better [ɑ̃n.n]) instead of the usual [an]. This used to be the standard French pronunciation before this nasalisation disappeared a few centuries ago.
For example, grammaire and grand-mère used to be pronounced identically.
— Veux-tu toute ta vie offenser la grammaire ?
— Qui parle d'offenser grand'mère ni grand'père ?
Molière, Les femmes savantes, acte II, scène 6, 1672.
And this might still be the case with some native speakers:
Another typical example is année sometimes pronounced [ɑ̃n.ne] instead of [ane].
With panneau, you might hear rares [panno] is southern France, but [pɑ̃n.no] is not expected.
The two are pronounced the same. The nasal sound might come from the double 'n', but that's beyond my expertise (and the question).
Reference : french is my mother tongue.
Petite vidéo avec les deux prononciations.