The sensor's Block mode detects only solid blocks, not fluids. I suggest that the sensor should be able to detect fluids as well, either in Block mode or a new fluid mode. The user would use filled buckets to choose which fluid to detect. If it's not too difficult, maybe the user could also use some modded containers such as IC2's fluid cells.

One possible use case is an automatic obsidian farm. The modpack on my server doesn't have any easy, automatic ways to generate obsidian without using a block breaker - that I can find, anyway - so I used block and fluid placers and block breakers from Actually Additions. The block placer prevented the water from flowing in front of the lava placer and blocking it before it could place lava. I eventually let the block placer and breaker work whenever they felt like it, but their timing was not very good and negatively affected the farm's productivity. Using a timer just reduced the farm's productivity further. Using a sensor to detect the presence of lava would have made the farm more productive and more robust against potential lag.

If you're interested, I made a prototype which supports detecting fluids in Block mode and tested it by building a simple obsidian farm in creative mode.