I was able to reproduce the issue in a Stein devstack setup,
(put a time.sleep(180) command in cinder.volume.drivers.lvm, method terminate_connection).

stack@fpalin-devstack:~$ openstack server list
+--------------------------------------+----------+--------+---------------------------------------------------------+-------+---------+
| ID                                   | Name     | Status | Networks                                                | Image | Flavor  |
+--------------------------------------+----------+--------+---------------------------------------------------------+-------+---------+
| aa68525e-6541-404a-bde1-5d21dbc3b1fb | test_VM3 | ACTIVE | private=fd3a:ef1e:3d05:0:f816:3eff:fe92:4f1d, 10.0.0.20 |       | m1.nano |
+--------------------------------------+----------+--------+---------------------------------------------------------+-------+---------+
stack@fpalin-devstack:~$ 
stack@fpalin-devstack:~$ openstack volume list
+--------------------------------------+------+--------+------+-----------------------------------+
| ID                                   | Name | Status | Size | Attached to                       |
+--------------------------------------+------+--------+------+-----------------------------------+
| ece0973b-9940-4849-a99a-71e6b62e46f7 |      | in-use |    1 | Attached to test_VM3 on /dev/vda  |
+--------------------------------------+------+--------+------+-----------------------------------+
stack@fpalin-devstack:~$  


stack@fpalin-devstack:~$ openstack server delete test_VM3
stack@fpalin-devstack:~$  
stack@fpalin-devstack:~$ openstack server list
+--------------------------------------+----------+--------+----------+-------+---------+
| ID                                   | Name     | Status | Networks | Image | Flavor  |
+--------------------------------------+----------+--------+----------+-------+---------+
| aa68525e-6541-404a-bde1-5d21dbc3b1fb | test_VM3 | ACTIVE |          |       | m1.nano |
+--------------------------------------+----------+--------+----------+-------+---------+
stack@fpalin-devstack:~$  openstack volume list
+--------------------------------------+------+--------+------+-----------------------------------+
| ID                                   | Name | Status | Size | Attached to                       |
+--------------------------------------+------+--------+------+-----------------------------------+
| ece0973b-9940-4849-a99a-71e6b62e46f7 |      | in-use |    1 | Attached to test_VM3 on /dev/vda  |
+--------------------------------------+------+--------+------+-----------------------------------+
stack@fpalin-devstack:~$ 


############## then after waiting about 3 minutes, volume remains attached, and stays that way:
stack@fpalin-devstack:~$ openstack server list

stack@fpalin-devstack:~$ openstack volume list
+--------------------------------------+------+--------+------+---------------------------------------------------------------+
| ID                                   | Name | Status | Size | Attached to                                                   |
+--------------------------------------+------+--------+------+---------------------------------------------------------------+
| ece0973b-9940-4849-a99a-71e6b62e46f7 |      | in-use |    1 | Attached to aa68525e-6541-404a-bde1-5d21dbc3b1fb on /dev/vda  |
+--------------------------------------+------+--------+------+---------------------------------------------------------------+
stack@fpalin-devstack:~$