I would be concerned for the spindle bearings. From your photo is looks like things got hot enough to discolor the collet? If so then I expect the bearings would have got very hot as there's not much distance between the collet and the bearings.

The bearings on my spindle do have seals, although due to being high speed bearings they rely more on the labyrinth seal. These pictures should give you an idea what to expect:

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If this had happened to me I'd definitely dismantle the spindle and have a look at the bearings. It's not too difficult to disassemble the spindle - the only difficult bit is removing the nut on the collet end. You might be fine as it is, or you might find the oil/grease has burnt away and the bearings fail in a few months.