Can android smartphones (with Wi-Fi enabled) be detected by other WiFi devices? So that the other Wi-Fi device knows an android phone (with wi-fi enabled) is nearby?
When you say that Wi-Fi is enabled I'm taking you literally: that Wi-Fi is enabled but is not actually connected to an access point (hotspot).

So the answer is no.

Just enabling the Wi-Fi on your phone does not make it visible to other devices.  It just sits there listening passively.  No one around you can see your wi-fi or get it to respond.

If you set your device to be a Wi-Fi hotspot (typically for tethering purposes) it will emit a signal called a beacon that allows other devices to see it.

Also, if you turn on Wi-Fi Direct then it can be seen, but this is a new feature in Android 4 that is not widely used yet and I don't know much about it.