A modular sensor activated faucet assembly (100) provides a spout (110) that can be coupled and removed from its mounting base (120) for installation and service. A water tight connection can be established between the mounting base and the spout by plugging the spout into the base. A seamless spout construction defining a hollow interior bifurcated by integral internal wall structure provides a wet chamber between its mounting end and the outlet and a flow pipe (103) inside the spout. The modular base provides a cooperating flow pipe (127). When the spout is mounted onto the base, the flow pipes (103, 127) are configured to nest together in close relation such that one or more seals (123) can be disposed between the flow pipes to provide a water tight seal between the spout and base.