If it happens during a general downtrend, it can mean the stock is "testing" itself. The fact that it occurs on low volume suggests it will only be a temporary rise and the downtrend will continue before long. The volume is low because the majority of investors watching the stock don't have much confidence in the upward spike, so they are not buying more. The big-money, experienced investors are waiting for the stock to reach its real bottom before buying. When that happens, you will see a rise on high volume that is more likely to be a true reversal, meaning the stock is going to continue rising.
Be sure to check other indicators as well, such as the RSI. If the RSI is over 70, the stock is probably overbought - another indication that a sell-off (drop) is imminent. As with all signs, these should be taken in the context of other indicators and the market in general. Hope this helps.