Small electrolytic capacitors (diameter 5 and 6.3 mm) usually don't have vents because:
- It's hard to manufacture a reliable vent on such small surface
- They don't generate enough energy (pressure) to be dangerous
- The bottom sealing is weak enough to blow relatively early in case of failure
As said before many axials, old or cheap electrolytics don't have vents.
Big screw terminals electrolytics have vents, it is generally a small hole with a rubber plug, not a weak point engraved in the housing.
You should not underestimate the risk of a capacitor blowing.
I've seen medium sized ones (diam 16-18 mm) make a dent in a steel 2 mm chassis, the same dent you can make by hitting it very hard with a hammer.
Even if there is no fire or projection, the noise of a big capacitor blowing can damage your hearing if you are too close.